Why turning on the spot, and what it costs

Zero-radius turning, tyre scrub, and the wear it produces.

A machine that can turn on the spot, driving one side forward and the other side backward instead of steering through a gentle arc, buys itself the ability to work in spaces a machine needing room to swing around a normal turning circle could never fit into, and it pays for that ability every single time it uses it, in tyre wear, ground damage and extra driving force that a wide, rolling turn never demands at all.

Why turning in place is not simply a tighter version of a normal turn

A wheel turning through an ordinary steered arc rolls cleanly along a curved path, its own rotation matching the actual distance it travels across the ground at every point, which is exactly what a rolling wheel is built to do efficiently. A wheel forced to turn the machine on the spot instead, spinning forward while the wheel on the opposite side spins backward, cannot roll cleanly at all, because the geometry of a genuine zero-radius turn asks each wheel to travel along a curved path centred on the machine's own middle, and a wheel simply is not shaped or oriented to roll naturally along a path like that, it has no choice but to slide sideways across the ground to some degree even while it continues turning. That sideways sliding is friction doing work against the ground rather than the wheel's own rotation carrying the machine anywhere useful, and it is the direct source of everything this kind of turn costs, in tyre wear, in ground damage, and in the extra driving force needed to keep the wheels turning against that resistance at all. A wheel further from the machine's own centre of rotation has to slide sideways across a proportionally larger arc for the same turn than a wheel sitting closer to that centre, which is why a longer machine, with its outermost wheels sitting further from the pivot point, tends to pay a noticeably higher scrubbing cost for the identical zero-radius turn than a shorter, more compact one does. The wheel sitting nearest the pivot is not spared entirely either, since even a wheel very close to the machine's centre still has to slide rather than roll through its own small arc, and the total cost of the turn is really the sum of every wheel's individual sliding contribution added together rather than something that can be traced back to any single wheel's behaviour on its own.

The pivot-turn comparison

Turning around on the spot by planting one foot and twisting the whole body around it leaves a visible scuff mark on a gym floor and a real scrape of extra wear on the sole of that planted shoe, a small but genuine amount of friction and abrasion a person can both feel through their foot and hear as a distinct scraping sound against the floor. Walking the same one-hundred-and-eighty-degree turn instead, in a wide, unhurried loop rather than a tight pivot, produces neither the mark nor the sound, since each foot in a walked turn simply lifts, rolls and sets back down again naturally with every step, carrying the body's weight only briefly and never dragging sideways against the floor while fully loaded the way the pivoting foot does for the whole of its twist. A machine turning on the spot is doing exactly what the pivoting foot does, holding its full weight on wheels that are being dragged sideways rather than rolled, and the scuff mark left on a gym floor by one twisting foot is a fair, if much smaller, preview of what a whole machine's tyres leave behind on real ground after the identical kind of turn. A person pivoting on a soft surface like sand rather than a hard gym floor feels the same trade even more directly, since the foot digs in and displaces material with every twist instead of merely scraping across something solid, which is closer to what happens under a machine's tyres on loose or wet ground than the cleaner scuff left on a hard floor.

Why the wear concentrates exactly where the turn happens

Unlike ordinary straight-line wear, which spreads itself out across an entire tyre's working life over however many kilometres the machine eventually covers, the sideways scrubbing from a zero-radius turn concentrates its damage into the specific, narrow patch of ground the machine happened to be pivoting on and the specific stretch of each tyre's tread that was in contact with the ground during that pivot. On soft or wet ground, this can tear and displace turf or soil badly enough to leave a visible, torn patch exactly where the machine turned, connecting directly back to the mud and ground-condition hazards described earlier in this archive's articles on outdoor equipment, since a machine that turns on the spot repeatedly in the same working area is actively degrading the very ground surface it depends on for traction with every turn it makes there. On a tyre itself, this same concentration shows up as an uneven wear pattern distinct from ordinary rolling wear, a telltale sign, on inspection, of a machine that spends a disproportionate share of its working life turning on the spot rather than driving in anything resembling a straight line. An experienced technician can often estimate roughly how much of a machine's working life was spent pivoting purely from the shape and location of that uneven wear pattern, well before ever consulting a usage log, since the tyre itself has been quietly keeping its own physical record of exactly how it was actually used. That record is also asymmetric between the two sides of the machine on any site where the same turn tends to happen in the same direction each time, since the wheels doing the reversing on a habitual left-hand pivot wear differently from the wheels continuing forward on the same turn, leaving a pattern a technician can read for which direction a machine favours as well as how often it turns on the spot at all.

The number that matters here

Turning a skid-steered machine on the spot can demand several times the driving force needed to move the identical machine forward in a straight line at a comparable speed, since almost all of that extra force is being spent dragging tyres sideways across the ground against real friction rather than driving the machine anywhere, a cost paid in full on every single pivot regardless of how briefly the turn itself takes.

My key error with this

We had predicted how the machine would behave on loose ground from the geometry and the specifications, which produced numbers that were reasonable, defensible and not based on the machine ever having been on loose ground. Running a proper set of tests in sand, with the conditions written down and the results measured rather than observed, gave us a completely different and far more useful picture, since we came away knowing what tyre pressure the machine actually wanted, which surfaces it could work on and which it could only cross, and roughly how often the wearing parts would need attention when it did. The most valuable part was knowing where the edges were, because a machine with a known envelope can be sent out confidently, while a machine with an estimated one gets treated cautiously everywhere and still surprises you occasionally. What replaced the belief is that predicted performance is a hypothesis until somebody drives the thing on the surface it will live on, and that quantified testing on real ground buys operational confidence that no amount of specification review will.